Time delay extractor fans - anyone opened one up?

[Mike Phelan]

Hi folks

Some bathroom extractor fans have a delay that keeps them running for a while after being switched off.

I've seen them at places like Wickes and B&Q where there are optional additions to a basic fan - timers, humidistats and the delay that I've mentioned. You can specify one, some or all of these with a price increase.

We do have a basic mains-driven fan, and as we're refurbing the bathroom next year, I was going to add a delay using scrapbox items like a triac, reed switch & coil, 555, wall wart and a few odds 'n' sods.

I've no problem designing such a gadget, but often wondered what the commercial ones used; presumably they have to be in close proximiy to the fan itself though my gadget could go anywhere.

Does anyone know how the commercial ones work?

TIA

[AndiiT]

Hi,
We have a time delay extractor fan in our bathroom, I have had the thing on and off the wall over the years for cleaning/decorating etc. from memory the circuit board housed within the fan charges a capacitor up when first switched on and a transistor/triac(?) combination maintains the power to the fan when the bathroom light is switched off until the capacitor discharges.

To be honest it takes a long time to stop after the light has been switched off and I did once open it up with the aim of modifying the circuit to reduce the on time. There is a pre-set which sets the delay time but it is now set at minimum and I didn't want to mess around with the circuitry too much as the house was rented at the time; I am told that the commercially designed fans were intended for use in a bathroom which has no windows, hence the long on time.

It's a bit dark this evening to try and remove the fan but I am happy to take it down in the morning, take a closer look, see if I can work the circuitry out and post some pictures for you.
I don't recall there being a relay fitted in the one that we have here but if you wanted your delay circuit to be external to the fan a relay may be the best option.

Regards
Andrew

[ppppenguin]

I'd be tempted to get one with a humidistat. Then it will run when needed and not otherwise. I can't say from first hand experience if they work as well as they ought. The time delays are a nuisance. You don't want it to run on after a brief visit but if you set the delay too short it may not be enough after a steamy bath.

[Alistair D]

I have repaired a few of them with both a 555 and a 4000 series cmos timer. All of them were powered by a capacitor dropper.

The usual fault is that to get the timer delay required the timing cap is usually electrolytic and the resistor is megs. Any leakage in the cap holds the fan on permanently.

Al

[Tractorfan]

Hi,
Personally, I wouldn't go with a humidistat as we had one in our kitchen extractor fan when we lived in the UK. And it was a BY nuisance. It used to kick in at odd times if it sensed the slightest bit of moisture then ran for hours with a really annoying whine. It drove us bonkers!! . In the end I ripped out the sensor circuitry and fitted a simple pull switch.
UK building regs require a timed fan if there's no other ventilation. If you've got an opening window, for example, you don't need a timer (so I'm told). We've got a timed fan here and there seems very little in it. The motor is switched by something that looks just like a plastic general purpose transistor (but can't be. Can it?)
Hope this helps
Cheers, Pete

[Herald1360]

Never had a look in a commercial unit, but 20 odd years ago, being a cheapskate, I built a unit to add delay to a simple fan. Can't say it was a cost effective solution but the bits I used were free- ISTR a 4060 oscillator divider generated the delay timing and the fan was switched by something like a 20A solid state relay (wouldn't do to underrate things)

Power was from a 12VA transformer bridge rectifier and a big C.

The whole lot sat in a diecast box on the wall alongside the fan; it was still there when we sold the house 5 years ago. I've no idea what your average sparky would make of it!

[Hunts smoothing bomb]

They use triacs, I have hundreds of them at the school boarding houses and when I get a job card saying "fan not working" I open it up to find the triac blown to pieces! Happens alot does this.

Cheers

[AlanBeckett]

Originally Posted by Tractorfan

... Personally, I wouldn't go with a humidistat as we had one in our kitchen extractor fan ...

I think things have improved. We had one installed in our main bathroom a few weeks ago and it just works. You start the shower and within a minute or so it kicks in. When you stop showering it stops after a few minutes. I've never heard it start at other times. The older one, installed in another shower room, which has a separate controller, is more problematic, although again it doesn't kick off randomly, it's just much slower to react.
I think yours was duff
Alan

[llama]

If I've remembered correctly, I bought a "timer" one when I only wanted a non-timer one. Again, if I've remembered correctly, I took out the delay module.

If so, I'll look around and you can have the delay module if it turns up.
Regards
Graham

[richrussell]

We have a humidistat one in our bathroom too - was put in a year ago. Generally it works well, coming on after about 2 minutes of the shower running, and going off again about the time the mirrors start to demist 15 minutes after switching the shower off - longer in winter.

However we do have to adjust the knob on the bottom that sets the level of humidity that it comes on at every couple of months, or it may not come on at all, or come on at odd times (like the middle of the night if it starts raining).

We do however live in a 1920's design house without central heating, which means the bathroom temperature and humidity vary quite widely throughout the year. Maybe in a more modern, better insulated house it'd be more reliable.

Overall though, it does work well - and has a manual override pull switch if you need it.

[valveaudio]

I am building one for my cloakroom. It will use a 555 triggered by a float switch in the cistern, so that you do not have to turn anything on.
Another alternative is to use a movement detector (no pun intended) to trigger the 555, probably using another 555 to prevent repetitive starts.

The 555 will operate a relay to switch on the fan.

Trevor

[Station X]

Mike.

I scrapped a bathroom fan back in January, but retained the circuit board. You're welcome to it FOC.

[llama]

There - 2 offers!

Mine's turned up - it's got a Y1112 power device and a TAGsomething TO92 device.
Mike (or anyone else) is welcome to it FOC.

Graham

[Mike Phelan]

Thanks, everyone, especially for the two Grahams.
Laziness intervenes so I won't be making one; it's coin-tossing time ........ Heads.
Graham Station X, PM on way in a few minutes.

Jeffrey - it won't be a problem as I'm wiring the fan to a pull switch that feeds the shower, which has its own switch. So I can turn the fan on and off at will, with or without using the shower.

If I needed a humidistat I already have a couple from scrapped dehumidifiers; we have one of those in the house and one in the workshop-cum-garage, and I find them 100% reliable. Consists of what looks like a cellophane strip which stretches when its humid.

[matthewhouse]

Originally Posted by Mike Phelan

Jeffrey - it won't be a problem as I'm wiring the fan to a pull switch that feeds the shower, which has its own switch. So I can turn the fan on and off at will, with or without using the shower.

I hope you use the correct size cable and appropriate fusing.

[Mike Phelan]

Originally Posted by matthewhouse

I hope you use the correct size cable and appropriate fusing.

It's already there, Matthew, and has been for a decade or two!

I think it's time to switch the thread off now, with no time delay.
Thanks to all.